Semi-blind transmit antenna array device using feedback information and method thereof in a mobile communication system

ABSTRACT

There is provided a transmit antenna array device with at least two antennas and a method thereof in which a transmission beam is appropriately formed based on a weight vector to be transmitted to a specific mobile station in a mobile communication system. For this purpose, a base station device has a reverse processor for processing a reverse signal received through the antenna array, a forward fading information extraction unit for extracting forward fading information from the received reverse signal, a beam formation controller for generating a weight vector for formation of a transmission beam using the forward fading information and the received reverse signal, and a forward processor having a transmission beam generator for generating a transmission beam for a transmission message based on the weight vector. A mobile station device has a forward processor for processing a received forward signal, a forward fading estimator for estimating forward fading information of the forward signal for each path, a forwarding fading encoder for combining the estimated forward fading information and encoding the combined forward fading information, and a reverse processor for multiplexing the encoded forward fading information with a transmission message and feeding back the forward fading information in the multiplexed signal to a base station.

PRIORITY

This application is a Divisional of U.S. application Ser. No.09/802,165, filed on Mar. 8, 2001, which claims priority to anapplication entitled “Semi-Blind Transmit Antenna Array Device UsingFeedback Information and Method Thereof in Mobile Communication System”filed in the Korean Industrial Property Office on Mar. 8, 2000 andassigned Serial No. 2000-11617, the contents of which are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to an antenna array device and amethod thereof in a mobile communication system, and in particular, to adevice and method for forming a transmission beam.

2. Description of the Related Art

As the number of mobile subscribers drastically increases, the capacityof the mobile communication systems approaches a saturation point. Thismeans that mobile communication systems are in need of more advancedapplications to increase the system capacity, particularly the capacityof a forward link for diverse multimedia services.

The capacity of the forward link can be increased by designing anefficient transmission antenna array system. If the mobile systems useonly single transmit antennas, for example dipole antennas, transmissionsignals are propagated in all directions. In this situation, when atransmission is performed toward a desired specific mobile stationthrough a dedicated channel, as opposed to a situation wheretransmission to all mobile stations is performed using a base stationtransmission antenna through a common channel, much of the radiationenergy except radiation energy for the specified mobile station isuseless and the unnecessary radiation energy becomes interferencesignals to other mobile stations. If the base station transmits a signalonly in the direction of the specific mobile station for communicationon the dedicated channel, good communication quality is ensured with lowtransmission power and interference to other mobile stations isdecreased. Consequently, the capacity of the forward link increases.

This effect can be achieved using a plurality of antennas. Atransmission/reception device related with the antennas is called atransmission antenna array system or transmission smart antenna system.While the transmit antenna array system is applicable to various mobilecommunication fields, it will be described here in context with CDMAcellular mobile communication for convenience sake.

The structure and operation of a transmit antenna array in the mobilecommunication system will be described hereinbelow.

FIG. 1 illustrates the transmission beam formation in the transmitantenna array. Referring to FIG. 1, let a transmission signal from abase station be s(t). The signal s(t) is duplicated into a plurality ofidentical signals, the duplication signals are multiplied bycorresponding complex weights in multipliers 111 to 11L, and theresulting signals are transmitted in the air through the respectiveantennas. A mobile station, using a single antenna, receives the sum ofthe transmission signals that the base station transmits through theantennas. A direction in which each transmission signal is directed isdetermined by a weight multiplied by the transmission signal and thegeometrical structure of the transmit antenna array. The reason forassuming that a single antenna is used in the mobile station is that themobile station does not typically use an antenna array due tolimitations of cost, size, and mobility.

Suppose a linear antenna array has L antennas as shown in FIG. 1 andeach antenna has a complex weight ω_(i) (i=1, 2, . . . , L), a signaltransmitted in a direction θ is proportional tow ^(H) a(θ)  (1)where w=[w₁w₂ . . . w_(L)]^(T) is a weight vector,${\underset{\_}{a}(\theta)} = \left\lbrack {1{\mathbb{e}}^{{j2\pi}\quad\frac{d\quad\sin\quad\theta}{\lambda}}\quad\ldots\quad{\mathbb{e}}^{{j2\pi}\frac{{({L - 1})}d\quad\sin\quad\theta}{\lambda}}} \right\rbrack^{T}$is an array vector, H represents Hermitian, T represents transpose, d isthe distance between antennas, and λ is the wavelength of a carrierfrequency. The array vector refers to the relative strength and phase ofa signal transmitted from each antenna to a remote destination in thedirection θ, as expressed in vectors.

w ^(H) a(θ) is greatest when w and a(θ) are in the same direction and w^(H) a(θ) is 0 when w is at a right angle with a(θ). Therefore, thestrength of a transmission signal varies according to the transmissiondirection θ. On the same principle, a signal can be transmitted with thegreatest strength in a specific direction θ by controlling w.

An antenna array is different from a diversity antenna device in that ittransmits a signal in a particular direction. The distance betweenantennas (wavelength order length) is shorter in the antenna array thanin the diversity antenna device.

In general, an antenna array is provided to a base station that canaccommodate a plurality of antennas and controls atransmission/reception direction with respect to a mobile station with asingle antenna. The antenna array can be considered in two parts: atransmission antenna array and a reception antenna array. Thetransmission antenna array is focused on for description by way ofexample. Yet, the hardware of the antenna array is commonly used fortransmission and reception.

A TDD (Time Division Duplex) system, since it uses an identicalfrequency band for transmission and reception, shows the samecharacteristics in transmission and reception and applies a weightvector obtained for a reception antenna array operation to atransmission antenna array operation as well. On the other hand, an FDD(Frequency Division Duplex) system calculates a weight vector separatelyfor a transmission antenna array because a transmission frequency bandis separated from a reception frequency band by a coherence bandwidth orgreater. It is to be appreciated that the following description is madeon a transmission antenna array system of an FDD system.

Blind transmission is characteristic of transmission antenna arrays thathave been developed so far. The blind transmission refers totransmission without receiving any feedback information of the channelcharacteristics of a forward link from a mobile station. Thesetransmission antennas operate based on the following reciprocitysuppositions between transmission and reception channels.

Supposition 1: a forward fading channel and a reverse fading channelarrive at their destinations from the same number of paths andtransmission and reception occur in the same path direction.

Supposition 2: if the difference between a transmission frequency bandand a reception frequency band is greater than a coherence bandwidth inan FDD system, the forward and reverse channels have mutuallyindependent instant fading coefficients but an identical average fadingpower for the same path.

Raleigh has suggested a blind transmit antenna array for a single fadingpath as shown in FIG. 2 (reference 1: G. G. Raleigh and V. K. Johnes,“Adaptive Antenna Transmission for Frequency Duplex Digital WirelessCommunication,” in Proc. IEEE ICC, pp. 641-646, Montreal, Canada, June1997).

A channel vector refers to a collection of the vector-expressedcharacteristics of each antenna in a transmit antenna array with respectto a reception antenna. If we let a forward channel vector be h, thenh=βa(θ). β is a fading coefficient independent of a reverse fadingcoefficient according to supposition 2, θ is a transmission directionfrom a base station to a mobile station, which the base station knowsfrom a reverse signal by supposition 1 without receiving forward fadingfeedback information from the mobile station, and a(θ)—corrected is anarray vector for the direction θ.

The base station transmits a transmission message s(t) by forming a beamwith a weight vector w and the message s(t) arrives at the mobilestation on a forward channel h. The received signal r(t) can beexpressed byr(t)= h ^(T) ws(t)+n(t)  (2)where n(t) is additive white Gaussian noise (AWGN).

According to a matching filter theory, an optimal weight vector thatbrings a maximal output SNR at a receiving end of the mobile station is$\begin{matrix}{\underset{\_}{w} = {\sqrt{P}\frac{{\underset{\_}{h}}^{*}}{\underset{\_}{h}}}} & (3)\end{matrix}$where P is the transmission power of the base station, * is a conjugateoperator, and ∥·∥ is the norm of a corresponding vector. By applying therelationship of h=βaθ to Eq. 3, $\begin{matrix}{\underset{\_}{w} = {\sqrt{P}{\frac{{\underset{\_}{a}}^{*}(\theta)}{{\underset{\_}{a}(\theta)}}.}}} & (4)\end{matrix}$

From Eq. 4, it is noted that an optimal weight vector is set using onlythe transmission direction θ known from a reverse signal by supposition1 without a fading coefficient. Because a single path is assumed, not afading coefficient but an array vector is obtained.

Now, a description of the transmission antenna array suggested byRaleigh will be given. Referring to FIG. 2, a transmission message ispropagated in the air via an antenna array 203 by a beam formed in aspecific transmission direction in a transmission beam generator 202. Areverse processor 205 processes a reverse channel signal received viathe antenna array 203. An array vector calculator 207 divides reverselyreceived signals for each path through a path divider in a rake receiverof the reverse processor 205 and calculates a direction (array vector)on the basis of direction information of the received signals. A weightvector calculator 209 calculates a weight vector using the array vectorand outputs the array vector to the transmission beam generator 202. Thetransmission beam generator 202 controls generation of a transmissionbeam by assigning a weight to a transmission signal that is to be outputvia a corresponding antenna based on the weight vector.

The above transmission antenna array system estimates the receptiondirection of a signal received via the antenna array 203, calculates aweight vector (array vector) for the transmission antenna array based onthe estimated direction information, and then forms a transmission beamusing the weight vector, for transmission.

Despite the advantage of simple structure, the Raleigh transmissionantenna array using a single path is not feasible for a multi-pathsystem.

Thompson has suggested a blind transmission antenna array with amulti-fading path as shown in FIG. 3 (reference 2: J. S. Thompson, J. E.Hudson, P. M. Grant, and B. Mulgrew, “CDMA Downlink Beamforming forFrequency Selective Channels,” PIMRC'99, B2-3, Osaka, Japan, September1999).

In the case of a multi-fading path (M paths), a reception direction foreach path must be estimated from an input signal to form a forwardtransmission beam as is done in the case of a single path. If areception direction (a transmission direction according tosupposition 1) for an ith fading path (i=1, 2, . . . , M) is θ_(i), atransmission beam for the ith fading path is formed in the direction ofθ_(i). The issue is how to determine weights (different from weightvectors). Considering this issue, an optimal weight vector is determinedin the following way.

Assuming that the base station transmits a transmission message byforming a transmission beam with a weight vector w and it arrives at themobile station from three different paths on a forward channel, a signalr(t) received at the mobile station can be expressed byr(t)= h ₁ ^(T) ws(t−τ ₁)+ h ₂ ^(T) ws(t−τ ₂)+ h ₂ ^(T) ws(t−τ ₃)  (5)where τ_(i) is a propagation delay for an ith path and h _(i) is achannel vector for an ith path. Similarly to a single path, with respectto the transmission direction θ_(i) and a fading coefficient β_(i), h_(i) is as follows. Herein, the fading coefficient β_(I) means a valueincluding a phase and a size value of the received signal.h _(i)=β_(i) a(θ _(i))  (6)

According to the matching filter theory, an optimal weight vector thatbrings a maximum output SNR at a receiving end of the mobile station is$\begin{matrix}{{{\underset{\_}{w}{^\circ}} = {{\arg_{w}^{\max}{\underset{\_}{w}}^{H}H^{H}H\underset{\_}{w}\quad{subject}\quad{to}\quad{\underset{\_}{w}}^{2}} = P}}{{{where}\quad H} = \left\lbrack {{\underset{\_}{h}}_{1}{\underset{\_}{h}}_{2}{\underset{\_}{h}}_{3}} \right\rbrack}} & (7)\end{matrix}$where P is transmission power, w° is an optimal weight vector, and h ₁,h ₂, h ₃ are channel vectors for the paths. The solution of Eq. 7 is setas a maximum unique vector corresponding to the maximum unique value ofa transmission correlation matrix${H^{H}H} = {\sum\limits_{i = 1}^{3}{{\beta_{i}}^{2}{\underset{\_}{a}\left( \theta_{i} \right)}{{\underset{\_}{a}\left( \theta_{i} \right)}^{H}.}}}$

From the foregoing, it can be noted that the base station needs to knowa fading coefficient {β_(i)} as well as a transmission direction {θ_(i)}in order to achieve the optimal weight vector. On the contrary, the basestation need not know a fading coefficient to form a transmission beamfor a single fading path. In an FDD environment, the instant fadingcoefficient of a reverse channel is different from that of a forwardchannel. Thus, it is no use analyzing a received reverse signal toobtain the instant fading coefficient of the forward channel.

By replacing H^(H)H of Eq. 7 with an expectation E[H^(H)H], Thompsonproposed a semi-optimal weight vector given by $\begin{matrix}{{E\left\lbrack {H^{H}H} \right\rbrack} = {\sum\limits_{i = 1}^{3}{{E\left\lbrack {\beta_{i}}^{2} \right\rbrack}{\underset{\_}{a}\left( \theta_{i} \right)}{{\underset{\_}{a}\left( \theta_{i} \right)}^{H}.}}}} & (8)\end{matrix}$

In Eq. 8, the transmission direction {θ_(i)} (the array vector{a(θ)_(i)}) is estimated from a received reverse signal according tosupposition 1 and E[|β_(i)|²] is also estimated from the receivedreverse signal according to supposition 2.

This is blind beam formation without the need of receiving feedbackinformation about a fading coefficient from a mobile station. However,the blind beam formation has a slightly lower performance than thenon-blind beam formation using an optimal weight vector calculated byEq. 7.

FIG. 3 is a block diagram of the transmit antenna array device suggestedby Thompson. Referring to FIG. 3, a transmission message is formed intoa beam by a transmission beam generator 302 of a forward processor 301and propagated in the air in a particular direction via an antenna array303. A reverse processor 305 processes a reverse channel signal receivedvia the antenna array 303. A forward fading power calculator 307estimates a fading coefficient of the received signal for each path,which is obtained by the reverse processor 305 in the course ofprocessing the received signals and calculates the average power of theestimated fading coefficients. The reverse average fading power iscalculated based on supposition 2. An array vector calculator 309divides the received signals for each path through a path divider in arake receiver of the reverse processor 305 and calculates the inputdirection (array vector) of the received signal from the receivedsignals. A transmission correlation matrix calculator 311 obtains atransmission correlation matrix using the average fading power and thearray vector and a weight vector calculator 313 calculates a weightvector using the transmission correlation matrix. The transmission beamgenerator 302 assigns a weight to a transmission signal that will beoutput via a corresponding antenna according to the weight vectorreceived from the weight vector calculator 313, to thereby controlformation of the transmission beam.

According to the Thompson transmit antenna array system, a receptionantenna array first estimates the input direction (array vector) of areceived signal. Then, the reception antenna array estimates a fadingcoefficient of the received signal for each path and calculates theaverage power of the fading coefficients. Based on the directioninformation and the average fading power information, a weight vectorfor a transmission antenna array is calculated. Finally, a transmissionbeam is formed using the weight vector and transmitted.

While the Thompson antenna array structure can be used as a transmissionantenna array system in a multi-path environment, the use of an averagefading power makes it impossible to calculate a precise weight vector.That is, the average fading power is used in calculating a forwardfading power instead of an instant fading power. The average fadingpower is calculated based on supposition 2. A reverse average fadingpower is calculated from a received signal for use as an average forwardfading power. The limitation of the Thompson antenna array incalculating a precise weight vector decreases the performance of theantenna array system.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a deviceand method for forming a transmission beam by calculating an optimalweight vector based on forward fading information feedback from a mobilestation in a base station using a forward antenna array in a mobilecommunication system.

It is also an object of the present invention to provide a device andmethod for estimating forward fading information from a signal receivedon a forward channel and transmitting the forward fading information toa base station on a reverse channel in a mobile station of a mobilecommunication system using an antenna array.

It is another object of the present invention to provide a transmitantenna array device and a method thereof suitable for a mobilecommunication system where a feedback delay time is short and a mobilestation roams at a movement speed that is not greatly changed.

It is a further object of the present invention to provide a transmitantenna array device and a method thereof in which a current forwardfading coefficient is estimated from previous forward fading informationfed back from a mobile station in a mobile communication system where afeed back delay time is great and the mobile station roams at a movementspeed that is not greatly changed.

It is still another object of the present invention to provide atransmit antenna array device using a mixed forward beam formationscheme where a basic type and a blind forward beam formation type areselectively used according to the movement speed of a mobile station anda method thereof when a feedback delay time is short in a mobile stationwith multiple signal paths.

It is yet another object of the present invention to provide an antennaarray device using a mixed forward beam formation scheme where aprediction type and a blind forward beam formation type are selectivelyused according to the movement speed of a mobile station and a methodthereof when a feedback delay time is rather long in a mobile stationwith multiple signal paths.

The foregoing and other objects of the present invention are achieved bya transmit antenna array device with at least two antennas and a methodthereof in which a transmission beam is formed appropriately based on aweight vector to be transmitted to a specific mobile station in a mobilecommunication system. For this purpose, a base station device has areverse processor for processing a reverse signal received through theantenna array, a forward fading information extraction unit forextracting forward fading information from the received reverse signal,a beam formation controller for generating a weight vector for formationof a transmission beam using the forward fading information and thereceived reverse signal, and a forward processor having a transmissionbeam generator for generating a transmission beam for a transmissionmessage based on the weight vector. A mobile station device has aforward processor for processing a received forward signal, a forwardfading estimator for estimating forward fading information of theforward signal for each path, a forwarding fading encoder for combiningthe estimated forward fading information and encoding the combinedforward fading information, and a reverse processor for multiplexing theencoded forward fading information with a transmission message andfeeding back the forward fading information in the multiplexed signal toa base station.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription when taken in conjunction with the accompanying drawings inwhich:

FIG. 1 illustrates transmission beam formation in a general transmitantenna array;

FIG. 2 is a block diagram of a conventional transmit antenna arraysystem suggested by Raleigh;

FIG. 3 is a block diagram of a conventional transmit antenna arraysystem suggested by Thompson;

FIG. 4 is a block diagram of a transmit antenna array system in a mobilecommunication system according to the present invention;

FIG. 5 is a block diagram of an embodiment of the transmit antenna arraysystem (a basic type) according to the present invention;

FIG. 6 is a block diagram of another embodiment of the transmit antennaarray system (a prediction type) according to the present invention;

FIG. 7 is a block diagram of a third embodiment of the transmit antennaarray system (a basic mixed type) according to the present invention;

FIG. 8 is a block diagram of a fourth embodiment of the transmit antennaarray system (a prediction mixed type) according to the presentinvention;

FIG. 9 is a flowchart illustrating the whole operation of the transmitantenna array system according to the present invention;

FIG. 10 is a flowchart illustrating a forward fading power calculationprocedure in the first embodiment of the transmit antenna array systemaccording to the present invention;

FIG. 11 is a flowchart illustrating a forward fading power calculationprocedure in the second embodiment of the transmit antenna array systemaccording to the present invention;

FIG. 12 is a flowchart illustrating a forward fading power calculationprocedure in the third embodiment of the transmit antenna array systemaccording to the present invention; and

FIG. 13 is a flowchart illustrating a forward fading power calculationprocedure in the fourth embodiment of the transmit antenna array systemaccording to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be describedhereinbelow with reference to the accompanying drawings. In thefollowing description, well-known functions or constructions are notdescribed in detail since they would obscure the invention inunnecessary detail.

The present invention uses an instant forward fading coefficient insteadof an average reverse fading coefficient in order to form a forward beamwith improved performance as compared to the conventional antenna arrayssystems. A base station, being a transmitting side, does not know theforward fading coefficient indicating channel characteristics inadvance. Thus, according to the preferred embodiment of the presentinvention, a mobile station extracts the forward fading coefficient fromthe forward channel information and feeds it back on a reverse channelto the antenna array system. Herein, the reverse channel fortransmitting forward channel information may be an existing reversechannel or a separately designated reverse channel. If an existingreverse channel transmits the forward fading coefficient, it may be acontrol channel. Then, a control channel message can be re-formatted toinclude the forward channel information.

While the conventional antenna array systems using an average reversefading coefficient rely on blind beam formation, an antenna array systemaccording to the present invention is a semi-blind beam formation schemein that a base station receives feedback information about a forwardfading coefficient from a mobile station.

The transmit antenna array system of the present invention relying onsemi-blind beam formation will be described below.

Assuming that a fading channel is propagated in M different pathsbetween a base station and a specific mobile station, and the basestation transmits a transmission message s(t) to the mobile stationusing a transmit antenna array including L antennas, a signal r(t)received at the mobile station is $\begin{matrix}{{r(t)} = {{\sum\limits_{i = 1}^{M}{{\underset{\_}{h}}_{i}^{T}\underset{\_}{w}{s\left( {t - \tau_{i}} \right)}}} + {n(t)}}} & (9)\end{matrix}$where w is a weight vector assigned to the transmit antenna array, n(t)is AGWN, τ_(i) is a propagation delay for an ith path, and h _(i) is achannel vector for the ith path, given byh _(i)=β_(i) a (θ_(i))  (10).

As noted from Eq. 10, the channel vector h _(i) is a function of afading coefficient β_(i), a transmission direction θ_(i), and an arrayvector a(θ_(i)) for the ith path.

The signal r(t) received at the mobile station on a forward channel canbe divided into path components by a path divider and a collection ofthe path components r is expressed as $\begin{matrix}\begin{matrix}{\underset{\_}{r} = {\begin{bmatrix}{{\underset{\_}{h}}_{1}^{T}\underset{\_}{w}{s\left( {t - \tau_{1}} \right)}} \\{{\underset{\_}{h}}_{2}^{T}\underset{\_}{w}{s\left( {t - \tau_{2}} \right)}} \\\vdots \\{{\underset{\_}{h}}_{M}^{T}\underset{\_}{w}{s\left( {t - \tau_{M}} \right)}}\end{bmatrix} + \begin{bmatrix}n_{1} \\n_{2} \\\vdots \\n_{M}\end{bmatrix}}} \\{= {{H\underset{\_}{w}s} + {n.}}}\end{matrix} & (11)\end{matrix}$

In Eq. 11, H=[h ₁, h ₂ . . . h _(M)]^(T), n=[n₁n₂ . . . n_(M)]^(T), ands(t−τ₁)=s(t−τ₂)=. . . s(t−τ_(M)). Here, s(t−τ₁), s(t−τ₂), . . . ,s(t−τ_(M)) are termed s. It is assumed that the length of a symbol inthe received message is greater than any path delay.

Applying a matching filter theory to Eq. 11, a determination variable, amatching filter output is given by $\begin{matrix}\begin{matrix}{\hat{s} = {\left( {H\underset{\_}{w}} \right)^{H}\underset{\_}{r}}} \\{= {{{\underset{\_}{w}}^{H}H^{H}H\underset{\_}{w}s} + {{\underset{\_}{w}}^{H}H^{H}\underset{\_}{n}}}}\end{matrix} & (12)\end{matrix}$and an SNR for the determination variable is $\begin{matrix}{\gamma = \frac{{\underset{\_}{w}}^{H}H^{H}H\underset{\_}{w}}{\sigma_{n}^{2}}} & (13)\end{matrix}$where σ_(n) ² is the power of the AGWN.

The optimal weight vector w maximizes the SNR of the matching filteroutput at a receiver on the assumption that the transmission power is P.From the foregoing, a transmission correlation matrix can be obtained by$\begin{matrix}\begin{matrix}{G = {H^{H}H}} \\{= {\sum\limits_{i = 1}^{M}{{\beta_{i}}^{2}{\underset{\_}{a}\left( \theta_{i} \right)}{{\underset{\_}{a}\left( \theta_{i} \right)}^{H}.}}}}\end{matrix} & (14)\end{matrix}$

Therefore, calculation of the optimal weight vector falls intocalculation of a maximum eigen-vector corresponding to a maximumeigen-value of the above transmission correlation matrix in the end.

The transmission direction θ_(i), that is, the transmission array vectora(θ_(i)) in Eq. 14 is known by estimating the reception direction of aninput signal according to supposition 1 in the base station. However,information about the forward fading coefficient β_(i), that is, theforward fading power |β_(i)|², cannot be obtained from the input signalbut fed back from the mobile station on a revere channel.

In the present invention, the base station receives feedback informationabout a forward fading coefficient calculated by the mobile station on aseparately designated reverse channel. By the forward fadingcoefficient, the base station forms the transmission correlation matrixG of Eq. 14 and calculates the maximal unique vector of the transmissioncorrelation matrix G, so that it calculates a weight vector for use informing an intended forward beam.

To reduce the feedback constraint of the mobile station by half, thebase station may receive information about fading severity or fadingpower in a real value instead of a fading coefficient in a complex valuefrom the mobile station. While the present invention is described incontext with formation of a transmission beam based on feedbackinformation of a fading coefficient, the same effect can be achieved byreceiving the fading severity or fading power.

The mobile station estimates an input signal component for each paththrough a path divider and a fading estimator as in Eq. 11. If noisecomponents are excluded from Eq. 11 for convenience sake,h _(i) ^(T) w=β _(i) a(θ _(i))^(T) w   (15)the fading estimator functions to estimate not the forward fadingcoefficient itself but the product of the forward fading coefficient,the array vector, and the weight vector as shown in Eq. 15. Although itis ideal that the mobile station transmits the forward fadingcoefficient only on a reverse fading channel, in reality, the basestation receives information including the forward fading coefficient,the array vector, and weights. Hereinbelow, the information includingthe forward fading coefficient, the array vector, and the weights willbe referred to as “fading information”. The forward fading coefficientcan be replaced by a forward fading severity. The following descriptionis based on a calculation of a weight vector using a forward fadingcoefficient. Therefore, the fading estimator in the base station mustextract only the forward fading coefficient β_(i) from the feedbackinformation received from the mobile station.

The forward fading coefficient is extracted by two methods. One is touse an ominidirectional beam with a(θ_(i))^(T) w independent of θ_(i) asa transmission beam so that an estimated value of the fading estimatorbecomes a function of the forward fading coefficient β_(i) only. Themobile station may feed back this value on a separately designatedreverse channel. The other method is to extract the forward fadingcoefficient β_(i) using a known weight vector w used in transmitting aforward signal to the mobile station and θ_(i) estimable from a receivedsignal by simple arithmetic operation in the base station, upon receiptof feedback information of an estimated input signal component for eachpath, {β_(i) a(θ_(i))^(T) w} on a specific reverse channel from themobile station.

A time delay may be involved in feeding back the forward fadingcoefficient β_(i). If a time delay as long as a unit time D, for examplea slot, exists between the mobile station and the base station,inevitably, a current forward fading coefficient must be estimated frompreviously fed back forward fading coefficients. This problem can beovercome by linear prediction.

Now a description of the linear prediction for estimating a forwardfeedback coefficient received with a time delay will be given.

Suppose β_(i)[k] is a forward fading coefficient for an ith path at akth time point (the present time point). By a linear combination of Vfading coefficients, β_(i)[k−D], β_(i)[k−D−1], . . . β_(i)[k−D−V+1],β_(i)[k] is estimated to $\begin{matrix}{{{\hat{\beta}}_{i}\lbrack k\rbrack} = {\sum\limits_{v = 0}^{V - 1}{b_{v}{\beta_{i}\left\lbrack {k - D - v} \right\rbrack}}}} & (16)\end{matrix}$

-   -   where if a definition is given as b=[b₀, b₁ . . . b_(v−1)]^(T)        and β=[β_(i)[k−D]β_(i)[k−D−1] . . . β_(i)[k−D−V+1]]^(T), the        equation 16 is        To obtain a coefficient vector β, a value β which allows        E(β(k)−{circumflex over ( )}β)² to be a minimum value, should be        calculated. Thus, the coefficient vector b is        b=R⁻¹ p  (17)        according to the linear prediction method.

In Eq. 17, R=E[ββ ^(H)] and p=[β_(i)[k]β*]. A correlation coefficientbetween delayed fading coefficients, needed to calculate Eq. 17 iscalculated byE[β _(i) [k]β _(i) *[k−u]]=σ _(β) ² J ₀(2πf _(D) Tu)  (18)

-   -   where β_(i)[k] is a Fading Coefficient received at a kth time        point on a ith path, a σ_(β) ²=E[|β_(i)|²], f_(D) is a Doppler        frequency, J₀(·) is a Bessel function of the first kind of order        zero, and T is the length of a unit time.

If the base station receives feedback only information of a fadingseverity being a real value instead of a fading coefficient being acomplex value from the mobile station, it is advantageous to reduce thenumber of bit sent to a reverse channel. If |β_(i)[k]| is a forwardfading severity for an ith path at a kth time point (the present timepoint), it is estimated by a linear combination of V fading severitiesfed back from the mobile station before the unit time D, |β_(i)[k−D]|,|β_(i)[k−D−1]|, . . . , |β_(i)[k−D−V+1]|. Thus, an average of theforward fading coefficient is 0 but average of the fading severity forobtaining an absolute value is not 0. In view of the foregoing, theabove step using a fading coefficient being a complex value cannot beapplied without modification.

However, the above procedure is not applicable because the average ofthe forward fading severities is not zero. A zero-average forward fadingseverity can be defined asδ_(i)=|β_(i) |=E[|β _(i)|]  (19)and |β_(i)[k]| is estimated by a linear combination of δ_(i)[k−D],δ_(i)[k−D−1], . . . , δ_(i)[k−D−V+1] as $\begin{matrix}{{{{\hat{\beta}}_{i}\lbrack k\rbrack}} = {{\sum\limits_{v = 0}^{V - 1}{d_{v}{\delta_{i}\left\lbrack {k - D - v} \right\rbrack}}} + {E\left\lbrack {\beta_{i}} \right\rbrack}}} & (20)\end{matrix}$where if d=[d₀d₁ . . . d_(v−1)]^(T) and δ=[δ_(i)[k−D][δ_(i)[k−D−1] . . .[δ_(i)[k−D−V+1]^(T), the coefficient vector d is calculated byd=R⁻¹ p  (21)according to the linear prediction method.

In Eq. 21, R=E[δδ ^(T)] and p=E[δ_(i)[k]δ]. A correlation coefficientbetween delayed zero-average fading coefficients, needed to calculateEq. 21 is calculated byE[δ _(i) [k]δ _(i) [k−u]]=σ _(δ) ² J ₀ ²(2πƒ_(D) Tu)  (22)where σ_(δ) ²=E[|δ_(i)|²], f_(D) is a Doppler frequency, J₀(·) is aBessel function of the first kind of order zero, and T is the length ofa unit time. E[β_(i)] is obtained by time-averaging forward fadingseverity sample values for each path.

As the mobile station travels at a higher speed, the Doppler frequencyincreases and channel characteristics are quickly changed. As a result,the reliability of the present forward fading coefficient estimated bythe linear prediction is decreased and the whole system performance isdeteriorated. In this case, the whole system performance may improve byusing an average reverse channel fading coefficient based on blindtransmission rather than the current forward fading coefficient based onlinear prediction. That is, first, a Doppler frequency threshold is setto a predetermined value. Then, when the measured Doppler frequency isbelow the threshold, which means low mobile speed, the linear predictionmethod is selected since the linear prediction is_regarded as reliable.Otherwise, the blind transmission method is selected instead of thelinear prediction method since the linear prediction is regarded asunreliable for high mobile speed. The selective use of the linearprediction method and the blind transmission method is called a mixedmethod.

Formation of a transmission beam can be implemented in four embodimentsaccording to time delay and channel changes. The four embodiments of thepresent invention are termed a basic type, a prediction type, a basicmixed type, and a prediction mixed type, respectively. Commonly in thefour embodiments, a mobile station feeds back forward fading informationto a base station and the base station generates a weight vector basedon the forward fading feedback information to efficiently form atransmission beam. The four embodiments are very similar in structureand operation but they differ in the essence of a transmission beamformation algorithm, the configuration of a transmission correlationmatrix.

FIG. 4 is a block diagram of a transmit antenna array system for a basestation in a mobile communication system according to the presentinvention. A mobile station is also shown in the drawing. The transmitantenna array system receives a forward fading coefficient from themobile station and calculates an optimal weight vector based on theforward fading coefficient to efficiently form a transmission beam. Themobile station, after receiving a forward signal from the base station,estimates the forward fading coefficient and transmits it to the basestation on a predetermined reverse channel. A forward fading severitycan be used instead of the forward fading coefficient. FIG. 9 is aflowchart illustrating an operation between the base station and themobile station shown in FIG. 4.

Referring to FIG. 4, the base station is comprised of a forwardprocessor 400, an antenna array 405, a reverse processor 410, a forwardfading information extraction unit 421, and a beam formation controller420.

The forward processor 400 subjects a transmission signal to encoding andmodulation, and upconverts the frequency of the modulated signal to anRF signal. A transmission beam generator 403 forms a beam for theforward transmission signal. The forward processor 400 includes anencoder 401, a modulator 402, the transmission beam generator 403, andan RF module 404.

The antenna array 405 includes L antennas and propagates a beam in adirection determined by the transmission beam generator 403 of theforward processor 400.

The reverse processor 410 subjects an RF signal received via the antennaarray 405 to downconversion, demodulation, and decoding. The reverseprocessor 410 includes an RF module 411, a rake receiver 412 with Mfingers, a path divider, and a path combiner, and a decoder 413.

The forward fading information extraction unit 421 functions to extracta forward fading coefficient from the fading information of a signalreceived from the rake receiver 412 of the reverse processor 410. Here,a forward fading severity may be included in the fading informationinstead of the forward fading coefficient.

The beam generation controller 420 receives the forward fadingcoefficient from the forward fading information extraction unit 421, andarray vector, a reverse fading information and a Doppler frequencyinformation output from the rake receiver 412 and generates a weightvector to control formation of the transmission beam. The beam formationcontroller 420 has a forward fading power calculator 422, an arrayvector calculator 423, a transmission correlation matrix calculator 424,and a weight vector calculator 425. As many forward fading powercalculators 422 and array vector calculators 423 as signal paths must beprovided. Therefore, it is desirable to configure the forward fadingpower calculator 422 and the array vector calculators 423 correspondingto the respective fingers of the rake receiver 412. The beam formationcontroller 420 varies in the four embodiments of the present invention.

A reception beam generator (not shown) is disposed before or after ademodulator in each finger of the rake receiver 412. The modulator 402may be exchanged with the transmission beam generator 403 in positionwithin the forward processor 400. Since the embodiments of the presentinvention are implemented on the same principle, the present inventionshould be considered to incorporate the above possibilities therein.

The transmit antenna array system of the base station controls formationof a transmission beam according to forward fading information fed backfrom the mobile station. To allow for this operation, the mobile stationreceives a forward link signal from the base station, estimates theforward fading information, and feeds back the estimated fadinginformation the base station, which will be described referring to FIG.4.

An antenna array system is not commonplace in mobile stations.Therefore, the mobile station shown in FIG. 4 uses a single antenna 431.

A forward processor 430 in the mobile station processes a signalreceived from the base station on a forward link. The forward processor430 includes an RF module 432, a rake receiver 433 with M fingers, apath divider, and a path combiner, and a decoder 434.

A reverse processor 440 subjects a transmission signal to encoding andmodulation, and transmits a modulated signal to the base station on areverse channel. The reverse processor 440 includes an encoder 441, amultiplexer 442, a modulator 443, and an RF module 444.

A forward processor 450 estimates forward fading information from thereceived forward link signal, encodes the estimated forward fadinginformation, and feeds back the encoded forward fading information on aparticular reverse channel. The forward processor 450 includes a forwardfading estimator 451 and a forward fading encoder 452. Here, “fadinginformation” estimated by the forward fading estimator 451 refers toinformation containing a weight vector and an array vector for beamformation as well as a fading coefficient for a multi-path fadingchannel. As shown in Eq. 15, the fading information contains the forwardfading coefficient β_(i), the array vector a(θ_(i)), and the weight w.This is the difference between the fading information of the presentinvention and fading information conventionally used in the case when asmart antenna is not employed.

The forward processors 400 and 430 and the reverse processors 410 and440, except for the transmission beam generator 403, are the same instructure as the counterparts used in a general CDMA communicationsystem.

Upon receipt of a transmission message through the encoder 401 and themodulator 402 in the base station, the transmission beam generator 403forms a transmission beam with use of an appropriate initial weightvector w[0] received from the weight vector calculator 425. Thetransmission beam is radiated into the air via the RF module 404 and theantenna array 405.

The mobile station receives the forward signal via the single antenna431 and the RF module 432, divides, demodulates, and combines theforward signal according to the paths in the rake receiver 433, andrecovers a received message in the decoder 434.

The forward signal received at the mobile station is given as Eq. 9. Thechannel vector h _(i) for each path in Eq. 9 includes information abouta forward fading coefficient and an array vector as shown in Eq. 10.Therefore, forward fading information estimated by the forward fadingestimator 451 includes a forward fading coefficient, an array vector,and a weight as shown in Eq. 15. Since what the base station needs isβ_(i), the forward fading estimator 451 may extract only the forwardfading coefficient from the fading information and transmits it. Or theforward fading estimator 451 may estimate a forward fading severitybeing a real value instead of the forward fading coefficient being acomplex value. The forward fading coefficient facilitates linearprediction, whereas feedback of the estimated forward fading coefficientto the base station is a constraint to the mobile station. On the otherhand, despite the advantage of a relatively small feedback constraint,the forward fading severity makes the linear prediction quitecomplicated. Accordingly, the forward fading coefficient or the forwardfading severity can be selected adaptively to the situation.

The forward fading encoder 452 encodes the estimated forward fadinginformation and feeds the encoded signal to the multiplexer 442. Theencoded forward fading information may be transmitted on a reverse linkchannel separately designated or it may be inserted into a modifiedcontrol channel frame and transmitted on an existing control channel.

In operation, the forward fading estimator 451 in the mobile stationestimates a fading coefficient for each path from a forward signalreceived from the rake receiver 433. The forward fading encoder 452collects all forward fading information and encodes the forward fadinginformation. The multiplexer 442 multiplexes the encoded forward fadinginformation with an encoded transmission message and the resultingsignal is radiated into the air via the modulator 443, the RF module444, and the single antenna 431.

Though the mobile station returns to a reception mode, transmission andreception are virtually concurrent in the mobile station and the aboveprocedure is repeated. It is assumed here that a delay of a unit time D(usually a slot) is involved between forward transmission and reversetransmission.

The base station receives the reverse signal via the antenna array 405and the RF module 411 and subjects the received reverse signal todemodulation and decoding through the rake receiver 412 and the decoder413. During this operation, a reverse beam generator in the rakereceiver 412 generates a reverse beam, which will not be described here.

The beam formation controller 420 of the base station calculates weightvectors for formation of a next transmission beam. The followingprocedure is common to the first and fourth embodiments of the presentinvention.

The forward fading information extraction unit 421 extracts forwardfading feedback information from the intermediate output of the rakereceiver 412. The forward power calculator 422 calculates a forwardfading power {p_(i)} for each path, which is really applied to atransmission antenna array, based on the extracted forward fadinginformation and reverse fading information for each path and Dopplerfrequency information obtained during processing the reverse signal inthe rake receiver 412. Simultaneously, the array vector calculator 423calculates an array vector {a(θ_(i))} for each path based on informationabout reception directions obtained during the reverse signal processingin the rake receiver 412.

The transmission correlation matrix calculator 424 calculates atransmission correlation matrix$G = {\sum\limits_{i = 1}^{M}{p_{i}{\underset{\_}{a}\left( \theta_{i} \right)}{\underset{\_}{a}\left( \theta_{i} \right)}^{H}}}$using the forward fading powers {p_(i)} and the array vectors{a(θ_(i))}. The weight vector calculator 425 calculates a maximum uniquevector of the transmission correlation matrix, normalizes it, and setsthe normalized maximum unique vector as a weight vector wk to thetransmission beam generator 403 for the next transmission.

The forward processor 400 encodes and modulates a transmission messagethrough the encoder 401 and the modulator 402. The transmission beamgenerator 403 forms a transmission beam according to the weight vectorreceived from the weight vector calculator 425 to transmit the modulatedtransmission message. The transmission beam is upconverted in the RFmodule 404 and radiates into the air via the antenna array 405.

Though the base station returns to a reception mode, transmission andreception are concurrent in the base station and the above operationrepeats.

FIG. 9 is a flowchart illustrating the operations of the base stationand the mobile station shown in FIG. 4. Referring to FIG. 9, the basestation sets a time point and a weight vector to initial values (k=0 andw[0]) in step 601 and the mobile station is also initialized in step651.

In steps 603 and 605, the base station encodes and modulates atransmission message through the forward processor 400, forms atransmission beam based on the weight vector, and transmits thetransmission beam on a forward link. In steps 607 to 611, the basestation increases the time point k and waits for the unit time D untilit receives a receiver signal. While awaiting receipt of the reversesignal, the base station performs other operations.

Upon receipt of the forward signal in step 653, the mobile stationseparates the paths from which the forward signal arrives through theforward processor 430. In steps 657 to 661, the mobile stationdemodulates the forward signal for each path and combines thedemodulated signals through the forward processor 430 and decodes thecombined signal, thereby recovering a received message. In steps 663 to669, the mobile station estimates forward fading information for eachpath, encodes the estimated forward fading information, and transmitsthe encoded forward fading information on a reverse channel through thereverse processor 440. Then, the mobile station returns to step 653 toawait receipt of a next forward signal.

Upon receipt of the reverse signal in step 611, the base stationseparates the paths from which the reverse signal arrives through thereverse processor 410 in step 613. In step 621, the base stationcontrols the forward fading power calculator 422 to select one of thefour embodiments for generation of a weight vector. The base stationcontrols the array vector calculator 423 to estimate an array vector{a(θ_(i))} for each path in step 615.

Selection of the embodiments of the present invention depends on thelength of the feedback delay time D and the movement speed of the mobilestation. If the feedback delay time D is relatively short and the mobilestation travels at a low speed, a forward fading power is calculated bychoosing the first embodiment of the present invention, a basic type instep 623.

If the feedback delay time D is long and the mobile station travels at alow speed, the forward fading power is calculated by choosing the secondembodiment of the present invention, a prediction type 625.

If the movement speed of the mobile station exceeds a threshold, use ofthe basic type or the prediction type may deteriorate performancedrastically. In this case, a blind forward beam formation method can bea desirable candidate. Therefore, the third embodiment (a basic mixedtype) or the fourth embodiment (a prediction mixed type) can beselectively used according to the movement speed of the mobile station.In the basic mixed type, a choice made between the basic type and theblind forward beam formation method, and in the prediction mixed type, achoice is made between the prediction type and the blind forward beamformation method. In step 627, the forward fading power is calculated ina selected type according to the third embodiment of the presentinvention and, in step 629 it is calculated in a selected type accordingto the fourth embodiment of the present invention.

A detailed description of the operation of calculating the forwardfading power p_(i) for each path according to the first to fourthembodiments of the present invention will be given below.

After the array vector and forward fading power for each path arecalculated, a transmission correlation matrix G is calculated in step631. In step 633, a maximum unique vector of the transmissioncorrelation matrix G is calculated and normalized, thereby updating theweight vector for use in forming a transmission beam that transmits anext transmission message.

The mobile station estimates the forward fading information and feedsback it to the base station. Then, the base station generates a weightvector by estimating the forward fading information, a reverse fadingpower, and a Doppler frequency received on a reverse link and forms atransmission beam based on the weight vector. For formation of thetransmission beam, the forward fading power calculator 422 operatesaccording to one of the four embodiments according to the feedback delaytime D and the movement speed of the mobile station.

First Embodiment (Basic Type)

A transmit antenna array system according to the first embodiment of thepresent invention is used when the feedback delay time D is 0 orrelatively short and the mobile station travels at a low speed. Thistransmit antenna array system is referred to as a basic type. FIG. 5 isa block diagram of the transmit antenna array system according to thefirst embodiment and FIG. 10 is a flowchart illustrating a forwardfading power calculating operation according to the first embodiment.

Referring to FIGS. 5 and 10, the path divider 501 of the rake receiver412 separates a reverse signal for each path, the demodulator 502 ineach finger demodulates the reverse signal for each path, and the pathcombiner 503 combines all finger outputs appropriately in steps 711 and713. In step 719, the decoder 413 decodes the combined signal, therebyrecovering a received message.

Meanwhile, a forward fading decoder 511 obtains forward fadinginformation that was received from the mobile station with a delay ofthe unit time D, {β_(i) ^(F)[k−D]a(θ_(i))^(H) w} or {|β_(i)^(F)[k−D]a(θ_(i))^(H) w|} and a forward fading extractor 512 extracts aforward fading coefficient β_(i)[k−D] or |β_(i)[k−D]| from the forwardfading information in step 715. Here, {a(θ_(i))^(H) w} is a value thatthe base station can know in advance. F represents forward, k is thecurrent time point, and i is a path index (i=1, . . . , M). In step 717,the base station regards the forward fading coefficient β_(i)[k−D] or|β_(i)[k−D]| as received at the current time point despite the timedelay of D and each power calculator 509 calculates forward fading power{p_(i)}={|β_(i) ^(F)[k]|} for each path.

Each array vector calculator 423 calculates an array vector from thereverse signal received from the demodulator 502. Then the transmissioncorrelation matrix calculator 424 calculates a transmission correlationmatrix G using the forward fading powers and the array vectors. Theweight vector calculator 425 calculates a maximum unique vector from thetransmission correlation matrix G, normalizes it, and sets thenormalized maximum unique vector as a weight vector w[k] fortransmission at the next time point.

Second Embodiment (Prediction Type)

When the feedback delay time D is rather long, the prediction type canbe used which has a means for predicting the current forward fadingcoefficient from previous feedback forward fading information. Atransmit antenna array system according to the second embodiment of thepresent invention is shown in FIG. 6 and its operation is illustrated inFIG. 11. While any predictor may be used with the same effect for thistransmit antenna array system, it is assumed that a linear predictor isprovided.

Referring to FIGS. 6 and 11, the path divider 501 of the rake receiver412 separates a reverse signal for each path, the demodulator 502 ineach finger demodulates a corresponding reverse signal, and the pathcombiner 503 combines all finger outputs appropriately in steps 811 and813. In step 821, the decoder 413 decodes the combined signal, therebyrecovering a received message.

Meanwhile, the forward fading decoder 511 obtains forward fadinginformation received from the mobile station with a delay of the unittime D, {β_(i) ^(F)[k−D]a(θ_(i))^(H) w} or {|β_(i) ^(F)[k−D]a(θ_(i))^(H)w|} and the forward fading extractor 512 extracts a forward fadingcoefficient β_(i)[k−D] or |β_(i)[k−D]| from the forward fadinginformation in step 815. The extracted forward fading coefficient isstored in a memory 513.

The previous forward fading information is read from the memory 513. Agroup of V delayed forward fading coefficients {β_(i) ^(F)[k−D], β_(i)^(F)[k−D−1], . . . , β_(i) ^(F)[k−D−V+1]} or {|β_(i) ^(F)[k−D], β_(i)^(F)[k−D−1], . . . , β_(i) ^(F)[k−D−V+1]|} are formed from the previousforward fading information.

In step 817, each reverse fading estimator 506 estimates a reversefading coefficient {β_(i) ^(R)} for each path, each average powercalculator 507 calculates an average power of the reverse fadingcoefficients {E|β_(i) ^(R)|²} for each path, and each Doppler frequencyestimator 505 estimates a Doppler frequency {ƒ_(D,i)} for each path.

In step 819, each current forward fading estimator 508 receives theforward fading coefficient, the average reverse fading power and theDoppler frequency obtained by reverse fading estimation for each pathand estimates a current forward fading for each path, and each powercalculator 509 calculates forward fading power for each path. Thecurrent forward fading estimator 508 forms the group of V delayedforward fading coefficients {β_(i) ^(F)[k−D], β_(i) ^(F)[k−D−1], . . . ,β_(i) ^(F)[k−D−V+1]} or the group of V delayed forward fading severities{|β_(i) ^(F)[k−D], β_(i) ^(F)[k−D−1], . . . , β_(i) ^(F)[k−D−V+1]|} fromthe previous forward fading information read from the memory 513.

In the case of the forward fading coefficient group, the current forwardfading estimator 508 estimates the current forward fading coefficient{|β_(i) ^(F)[k]|} using {β_(i) ^(F)[k−D], β_(i) ^(F)[k−D−1], . . . ,β_(i) ^(F)[k−D−V+1]}, {E|β_(i) ^(R)|²}, and {ƒ_(D,i)} for acorresponding path by the linear prediction method shown in Eq. 16, Eq.17, and Eq. 18. On the other hand, in the case of the forward fadingseverity group, the current forward fading estimator 508 estimates thecurrent forward fading severity {|β_(i) ^(F)[k]|} using {β_(i)^(F)[k−D], β_(i) ^(F)[k−D−1], . . . , β_(i) ^(F)[k−D−V+1]}, {E|β_(i)^(R)|²}, and {ƒ_(D,i)} for a corresponding path by the linear predictionmethod shown in Eq. 20, Eq. 21, and Eq. 22.

The power calculator 509 calculates a forward fading power{p_(i)}={|β_(i) ^(F)|²} for the corresponding path based on the forwardfading coefficient.

The array vector calculator 423 calculates an array vector from thereverse signal of the corresponding path received from the demodulator502. Then the transmission correlation matrix calculator 424 calculatesa transmission correlation matrix G using the forward fading powers andthe array vectors. The weight vector calculator 425 calculates a maximumunique vector from the transmission correlation matrix G, normalizes it,and sets the normalized maximum unique vector as a weight vector w[k]for transmission at the next time point.

Third Embodiment (Basic Mixed Type)

When the feedback delay time D is 0 or relatively short, the firstembodiment shows good performance until the movement speed of the mobilestation reaches a threshold. However, once the mobile station travels ata speed over the threshold, the performance drastically decreases. Toovercome this problem, the blind forward beam formation method can beused when it is determined that the mobile station travels at a speedover the threshold. In the third embodiment, the basic type and theblind forward beam formation method are selectively used according tothe movement speed of the mobile station. This scheme is referred to asa basic mixed forward beam formation method.

FIG. 7 is a block diagram of a transmit antenna array system accordingto the third embodiment of the present invention and FIG. 12 is aflowchart illustrating the operation of a forward fading powercalculator in the transmit antenna array system according to the thirdembodiment of the present invention.

Referring to FIGS. 7 and 12, the path divider 501 of the rake receiver412 separates a reverse signal for each path, the demodulator 502 ineach finger demodulates a reverse signal, and the path combiner 503combines all finger outputs appropriately in steps 911 and 913. In step927, the decoder 413 decodes the combined signal, thereby recovering areceived message.

Meanwhile, the forward fading decoder 511 obtains forward fadinginformation received from the mobile station with a delay of the unittime D, {β_(i) ^(F)[k−D]a(θ_(i))^(H) w} or {|β_(i) ^(F)[k−D]a(θ_(i))^(H)w|} and the forward fading extractor 512 extracts a forward fadingcoefficient β_(i)[k−D] or |β_(i)[k−D]| from the forward fadinginformation in step 915. Here, {a(θ_(i))^(H) w} is a value that the basestation can know in advance. F represents forward, k is the current timepoint, and i is a path index (i=1, . . . , M). In step 917, the basestation regards the forward fading coefficient β_(i)[k−D] or|β_(i)[k−D]| as received at the current time point despite the timedelay of D and each power calculator 509 calculates a forward fadingpower {|β_(i) ^(F)|²} for each path.

Simultaneously, each reverse fading estimator 506 estimates a reversefading coefficient for each path from a reverse signal received from thedemodulator 502, each average power calculator 507 calculates an averagereverse fading power {E|β_(i) ^(R)|²} for each path, and each Dopplerfrequency estimator 505 estimates a Doppler frequency for each path instep 919.

In step 921, a selector 510 selects {|β_(i) ^(F)|²} or {E|β_(i) ^(R)|²}based on the Doppler frequency. Specifically, if the Doppler frequencyis less than a predetermined threshold, it is determined that themobility of the mobile station is low in step 921 and the forward fadingpower {|β_(i) ^(F)|²} is selected in step 923. On the other hand, if theDoppler frequency is greater than or equal to the threshold, it isdetermined that the mobility of the mobile station is high in step 921and the average reverse fading power {E|β_(i) ^(R)|²} is selected andoutput as {p_(i)} in step 925.

Each array vector calculator 423 calculates an array vector from thereverse signal of each path received from the demodulator 502. Then thetransmission correlation matrix calculator 424 calculates a transmissioncorrelation matrix G using the forward fading powers and the arrayvectors. The weight vector calculator 425 calculates a maximum uniquevector from the transmission correlation matrix G, normalizes it, andsets the normalized maximum unique vector as a weight vector w[k] fortransmission at the next time point.

Fourth Embodiment (Prediction Mixed Type)

When the feedback delay time D is rather long, the second embodimentshows good performance until the movement speed of the mobile stationreaches a threshold. However, once the mobile station travels at a speedover the threshold, the performance drastically decreases. To overcomethis problem, the blind forward beam formation method can be used whenit is determined that the mobile station travels at a speed over thethreshold. In the fourth embodiment, the prediction type and the blindforward beam formation method are selectively used according to themovement speed of the mobile station. This scheme is referred to as aprediction mixed forward beam formation method.

FIG. 8 is a block diagram of a transmit antenna array system accordingto the fourth embodiment of the present invention and FIG. 13 is aflowchart illustrating the operation of a forward fading powercalculator in the transmit antenna array system according to the fourthembodiment of the present invention.

Referring to FIGS. 8 and 13, the path divider 501 of the rake receiver412 separates a reverse signal for each path, the demodulator 502 ineach finger demodulates a corresponding reverse signal, and the pathcombiner 503 combines all finger outputs appropriately in steps 1011 and1012. In step 1027, the decoder 413 decodes the combined signal, therebyrecovering a received message.

Meanwhile, the forward fading decoder 511 obtains forward fadinginformation received from the mobile station with a delay of the unittime D, {β_(i) ^(F)[k−D]a(θ_(i))^(H) w} or {|β_(i) ^(F)[k−D]a(θ_(i))^(H)w|} and the forward fading extractor 512 extracts a forward fadingcoefficient β_(i)[k−D] or |β_(i)[k−D]| from the forward fadinginformation in step 1013. The extracted forward fading coefficient isstored in the memory 513.

Simultaneously, each reverse fading estimator 506 estimates a reversefading coefficient {β_(i) ^(R)} for each path from a reverse signalreceived from the demodulator 502, each average power calculator 507calculates an average reverse fading power {E|β_(i) ^(R)|²} for eachpath, and each Doppler frequency estimator 505 estimates a Dopplerfrequency {ƒ_(D,i)} for each path in step 1015.

In step 1017, each current forward fading estimator 508 receives theforward fading coefficient, the average reverse fading power, theDoppler frequency and estimates a current forward fading for each path.That is, each current forward fading estimator 508 reads the previousforward fading information from the memory 513 and forms a group of Vdelayed forward fading coefficients {β_(i) ^(F)[k−D], β_(i) ^(F)[k−D−1],. . . , β_(i) ^(F)[k−D−V+1]} or a group of V delayed forward fadingseverities {|β_(i) ^(F)[k−D], β_(i) ^(F)[k−D−1], . . . , β_(i)^(F)[k−D−V+1]|} from the previous forward fading information.

In the case of the forward fading coefficient group, the current forwardfading estimator 508 estimates the current forward fading coefficient{|β_(i) ^(F)[k]|} using {β_(i) ^(F)[k−D], β_(i) ^(F)[k−D−1], . . . ,β_(i) ^(F)[k−D−V+1]}, {E|β_(i) ^(R)|² }, and {ƒ_(D,i)} for thecorresponding path by the linear prediction method shown in Eq. 16, Eq.17, and Eq. 18. On the other hand, in the case of the forward fadingseverity group, the current forward fading estimator 508 estimates thecurrent forward fading severity {|β_(i) ^(F)[k]|} using {β_(i)^(F)[k−D], β_(i) ^(F)[k−D−1], . . . , β_(i) ^(F)[k−D−V+1]}, {E|β_(i)^(R)|²}, and {ƒ_(D,i)} for a corresponding path by the linear predictionmethod shown in Eq. 20, Eq. 21, and Eq. 22.

Each power calculator 509 calculates a forward fading power {|β_(i)^(F)|²} for each path based on the forward fading coefficient. In step1021, the selector 510 selects {|β_(i) ^(F)|²} or {E|β_(i) ^(R)|²} usingthe Doppler frequency. Specifically, if the Doppler frequency is lessthan a predetermined threshold, it is determined that the mobility ofthe mobile station is low in step 1021 and the forward fading power{|β_(i) ^(F)|²} is selected in step 1023.

On the other hand, if the Doppler frequency is greater than or equal tothe threshold, it is determined that the mobility of the mobile stationis high in step 1021 and {E|β_(i) ^(R)|²} is selected and output as{p_(i)} instep 1025.

Each array vector calculator 423 in the finger calculates an arrayvector from the reverse signal of each path received from thedemodulator 502. Then the transmission correlation matrix calculator 424calculates a transmission correlation matrix G using the forward fadingpowers and the array vectors. The weight vector calculator 425calculates a maximum unique vector from the transmission correlationmatrix G, normalizes it, and sets the normalized maximum unique vectoras a weight vector w[k] for transmission at the next time point.

In the mobile communication system with a transmission antenna arraydevice of the present invention as described above, since a mobilestation feeds back forward fading information to a base station, thebase station can form a transmission beam more reliably. As a result,system capacity is increased, communication quality is improved, and thetransmission power of the mobile station is saved.

While the invention has been shown and described with reference tocertain preferred embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims.

1. A mobile station device in a mobile communication system, comprising:a forward processor for processing a received forward signal; a forwardfading estimator for estimating forward fading information of theforward signal for each path; a forwarding fading encoder for combiningthe estimated forward fading information and encoding the combinedforward fading information; and a reverse processor for multiplexing theencoded forward fading information with a transmission message andfeeding back the forward fading information in the multiplexed signal toa base station, wherein if the forward signal forms an omnidirectionalbeam, the forward fading estimator estimates complex forward fadinginformation {β_(i) ^(F), i=1, 2, . . . , M}.
 2. The mobile stationdevice of claim 1, wherein if the forward signal forms anomnidirectional beam, the forward fading estimator estimates forwardfading severity information {|β_(i) ^(F)|, i=1, 2, . . . , M}.
 3. Themobile station device of claim 1, wherein the forward fading estimatorestimates complex forward fading information {β_(i) ^(F) a(θ_(i))^(H)w|, i=1, 2, . . . , M } from the forward signal.
 4. The mobile stationdevice of claim 1, wherein the forward fading estimator estimatesforward fading severity information {|β_(i) ^(F) a(θ_(i))^(H) w, i=1, 2,. . . , M} from the forward signal.
 5. The mobile station device ofclaim 1, wherein the reverse processor inserts the forward fadinginformation into a predetermined reverse channel message, fortransmission.
 6. The mobile station device of claim 1, wherein thereverse processor transmits the forward fading information on aseparately designated reverse channel.